Implementing ROPC based authentication in asp .net core - asp.net

Question

I'm trying to implement ROPC flow in asp.net api . Can you please guide me on how to achieve this properly in ASP.net core using built in authentication library. If it is not possible what are the alternatives
I'm able to get the access token using http call to azure AD token endpoint. But i'm not able to validate the token using built in authentication library
can we implement this without the HTTP call , but using built in library.
services.AddAuthentication(AzureADDefaults.BearerAuthenticationScheme)
.AddAzureADBearer(options => Configuration.Bind("AzureAd", options));
Im using the below code snippet to get the access token
private async Task<string> GetTokenAsync(string username, string password)
{
string grantType = "password";
string tenantId = Configuration["AzureAdNative:TenantId"];
string clientId = Configuration["AzureAdNative:ClientId"];
string resource = Configuration["AzureAdNative:Resource"];
string endpoint = "https://login.microsoftonline.com/" + tenantId + "/oauth2/token";
HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/x-www-form-urlencoded"));
var dict = new Dictionary<string, string>();
dict.Add("grant_type", grantType);
dict.Add("client_id", clientId);
dict.Add("username", username);
dict.Add("password", password);
dict.Add("resource", resource);
var req = new HttpRequestMessage(HttpMethod.Post, endpoint) { Content = new FormUrlEncodedContent(dict) };
var res = await client.SendAsync(req);
string result = res.Content.ReadAsStringAsync().Result;
var jwt = JObject.Parse(result);
return result;
// return jwt.GetValue("access_token").ToString();
}
getting unauthorised 401 error while validating with above code. access token is being sent in the authorization header (eg: Authorization : Bearer e4dgkddskdsdk).